It would help to know what what device you've got. Looks like a Lumia, but I can't tell the model number.
If it's got the AMOLED screen, it will run hot and you won't get much runtime off the battery. If it's LCD, it won't harm anything, but may shorten your battery life (more light, more power used). The AMOLED screens can burn in if left on the same image for days at a time, where an LCD won't. Generally, having it set to its highest setting, won't hurt anything.